Apparatus and method for measuring clinical-audiometric parameters

ABSTRACT

Apparatus and method for determining the immittance of a middle ear for clinical- audiometric investigations in a wide range of frequencies at ambient pressure, based on MEMS microphone technology and on measuring the acoustic pressure wave and the corresponding acoustic velocity wave by means of a pressure-pressure probe.

The present invention relates to an apparatus and method for determiningthe immittance of a middle ear for clinical-audiometric investigation ina in a simple, non-invasive, reliable and economical way.

Such invention is based on MEMS microphone technology and on thesimultaneous measurement of the acoustic pressure wave and thecorresponding acoustic velocity wave with pressure-pressure probe.

The present invention is mainly oriented to ear impedancemetry, alsocalled tympanometry, towards diagnostic applications for determining thestate of the tympanic membrane of a middle ear in humans or in anotheranimal equipped with an ossicular tympanic system and external auditorycanal, such as for example a dog or a cat.

Classical tympanometry allows to carry out an objective analysis of thefunctional state of the external and middle ear in order to diagnose themain pathologies. It consists in sending a mono-frequency sound wave,usually around 226, 678, 800 and 1000 Hz and evaluating the variationsin the acoustic volumetric impedance, measured at the tympanic membranelevel, as a function of pressure variations artificially created in theexternal auditory canal, usually included in a range between -600 and+400 daPa.

The classical tympanometric technique has disadvantages. In fact, aclassic tympanometric test is carried out with indirect measurements,using monofrequency stimuli, and with an invasive method, due to anoutside induced variation of the static pressure in the auditory canal,which is particularly annoying especially for infants and children.

As an alternative to the classical tympanometric technique, a broadbandtympanometric technique with p-v microprobes has also been developed,which detects the dependence of the specific admittance of the systemincluding the canal and the eardrum on the frequency of a sound wavewhich can vary from 50 Hz to 8 kHz, at constant static pressure. Thistechnique is based on microprobes p-v (pressure-velocity) with MEMStechnology (Micro Electro-Mechanical Systems) capable of directlymeasuring the values of pressure and velocity in response to an externalstimulus.

However, this technique also has disadvantages in that p-v microprobeshave high costs and, moreover, the velocity probe is fragile and easilysubject to breakage.

The object of the present invention is overcoming the describeddisadvantages, allowing to carry out clinical-audiometric investigationson the functional state of an ear in a reliable, non-invasive, andeconomical way.

The measurement uncertainty of the ear canal reflectance associated withthe conventional measurement technique was investigated by reproducingear canal reflectance measurements using two different measurementtechniques, as reported in Kren Monrad Nørgaard et al.: “Reproducingear-canal reflectance using two measurement techniques in adult ears”,The Journal Of The Acoustical Society Of America, American Institute OfPhysics For The Acoustical Society Of America, New York, NY, US, vol.147, no. 4, 17 Apr. 2020 (2020-04-17), pages 2334-2344, XPO12246238,ISSN:0001-4966, DOl: 10.1121/10.0001094.

It is specific subject matter of the present invention a method fordetermining the admittance of an auditory canal for clinical-audiometricinvestigations, the method comprising at least one or more iterations ofa procedure, in which each iteration is associated with a respectivecoupling configuration (Q, Q1, Q2) between an impedance probe and theauditory canal, in which said procedure includes the following steps:

-   A. coupling the calibrated sealed impedance probe having a known air    volume V_(probe), through a first end thereof, with the auditory    canal so that:    -   the air volume V_(probe) sealed inside the impedance probe and        the air volume V_(canal) inside the auditory canal form an        overall air volume V_(overall), and    -   a longitudinal axis of the impedance probe is substantially        coincident to a longitudinal axis of the auditory canal;-   B. sending a broadband exciting sound signal s(t) to the auditory    canal through a speaker of the impedance probe, such speaker being    located at a second end of the impedance probe opposed to the first    end;-   C. directly detecting an acoustic pressure p₁(t), p₂(t) back from    the auditory canal in at least two points x₁ and x₂, respectively,    located along the longitudinal axis of the impedance probe at    distance Δ×₁₂ between them, by means of a microphone array that is    included in the impendence probe and outputs electric signals r₁(t)    and r₂(t);-   D. acquiring and discretizing the output electric signals r₁(t) and    r₂(t) from the microphone array, obtaining discretized signals r₁(n)    and r₂(n) respectively, with n∈[1; N], N∈N;-   E. calculating a first impulse response-   δ₁^(au)(n)-   and a second impulse response-   δ₂^(au)(n)-   by the following equations:-   $\delta_{1}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{1}(t)} \right\}} \right\}$-   $\delta_{2}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{2}(t)} \right\}} \right\}$-   where s′(t) is the time reversed broadband sound signal s(t), FFT is    a Fast Fourier Transform and IFFT is an inverse FFT;-   F. calculating an impulse response-   δ_(p)^(au)(n)-   of the acoustic pressures p₁(t) and p₂(t) and an impulse response-   δ_(v)^(au)(n)-   of velocity of an air particle at measurement point x₀ along the    longitudinal axis of the impedance probe:-   $\delta_{v}^{au}(n) = \frac{\delta_{1}^{au}(n) - \delta_{2}^{au}(n)}{\left( {\rho \cdot \Delta\text{x}_{12}} \right)} + \delta_{v}^{au}\left( {n - 1} \right)$-   $\delta_{p}^{au}(n) = \frac{\delta_{1}^{au}(n) + \delta_{2}^{au}(n)}{2}$-   such a measurement point x₀ being a centre point between points x₁    and x₂;-   G. converting the impulse responses-   δ_(p)^(au)(n),-   δ_(v)^(au)(n)-   of pressure and velocity to pressure and velocity physical units by    multiplying each one by a calibration constant α and β known a    priori, respectively, as follows:-   δ_(p)(n)[Pascal] = α ⋅ δ_(p)^(au)(n)-   δ_(v)(n)[Pascal meter/second] = β ⋅ δ_(v)^(au)(n)-   H. calculating (270) frequency spectra P̂*(ω_(m)), V̂*(ω_(m)) of the    impulse responses of pressure and velocity respectively, through    Fast Fourier Transform, as follows:-   $\left\{ \begin{array}{l}    {{\hat{V}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{v}(n)} \right\}} \\    {{\hat{P}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{p}(n)} \right\}}    \end{array} \right)$-   where ω_(m) is a discretized frequency with m∈[1; N/2];-   I. calculating an admittance Ŷ*(ω_(m)) as a ratio between a cross    spectrum Ĝ_(pv)(ω_(m)) of the spectrum of the acoustic pressure    impulse response and of the spectrum of the acoustic velocity    impulse response, and an auto spectrum Ĝ_(pp)(ω_(m)) of the spectrum    of the acoustic pressure impulse response:-   ${\hat{Y}}^{\ast}\left( \omega_{m} \right) = \frac{{\hat{G}}_{pv}\left( \omega_{m} \right)}{{\hat{G}}_{pp}\left( \omega_{m} \right)} = \frac{{\hat{V}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}{{\hat{P}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}$-   L. obtaining a calibrated frequency spectrum Ŷ(ω_(m)) of the    admittance through a calibration function Γ(ω_(m)) known a priori,    according to the equation:-   Ŷ(ω_(m)) = Γ(ω_(m)) ⋅ Ŷ^(*)(ω_(m)) ,-   said steps D to L being performed by a control and processing    device.

According to another aspect of the invention, said calibration constantα and β, and the calibration function Γ(ω_(m)) can be known a priori,optionally provided by manufacturer of microphones.

According to a further aspect of the invention, said exciting soundsignal s(t) can be a sweep signal, optionally a linear or logarithmicsinusoidal signal, varying from a minimum frequency F_(min) greater than100 Hz to a maximum frequency F_(max) less then 5000 Hz over a timeT_(sweep) less than 10 seconds, optionally equal to 2 seconds, moreoptionally equal to 1 second.

According to an additional aspect of the invention, said distance Δx₁₂can be equal to 12 mm.

According to another aspect of the invention, said step B can comprisethe substeps:

-   B.1 synthesizing a digital sweep signal s(n) by a signal generator,-   B.2 converting the digital sweep signal s(n) into a broadband    exciting sound signal s(t) to be input to the speaker through a D/A    converter.

According to a further aspect of the invention, said step D can beimplemented through an A/D converter synchronized with the D/Aconverter.

According to an additional aspect of the invention, said calibratedfrequency spectrum Ŷ(ω_(m)) of the admittance can be further input to adisplay to be displayed.

According to another aspect of the invention, said impedance probe andsaid auditory canal can be coupled into a first coupling configuration(Q1), said exciting sound signal s(t) can be a fast sweep signals^(fast)(t) varying in frequency over a time

T_(sweep)^(fast)

less than one second, to obtain a first calibrated admittance Ŷ₁(ω_(m))and said procedure can comprise further the additional step:

-   M. checking whether a resonance condition in the calibrated    admittance Ŷ₁(ω_(m)) is satisfied, thereby a peak of the module of    the first calibrated admittance Ŷ₁(ω_(m)) corresponds to    zero-crossing of its phase, and wherein:    -   if a resonance condition does not occur, another iteration of        said procedure comprising steps A to M is implemented, wherein        the impedance probe and the auditory canal are coupled in        another coupling configuration (Q2) that is different from the        previous coupling configuration (Q1) and the exciting sound        signal s(t) is the fast sweep signal s^(fast)(t);    -   if a resonance condition occurs, steps B to L of such procedure        are implemented, wherein the coupling configuration is the one        for which the resonance condition occurs and the exciting sound        signal s(t) is a sweep signal varying in frequency over a time        greater than the time    -   T_(sweep)^(fast)    -   of the fast sweep signal s^(fast)(t), and said one or more        iterations of said procedure end.

It is another specific subject matter of the present invention aclinical-audiometric investigation method comprising the method fordetermining the admittance of an auditory canal according to thisinvention, wherein the investigation is implemented in the couplingconfiguration (Q, Q1, Q2) of the last one or more iterations and whereinthe exciting sound signal s(t) varies in frequency over a time grater orequal to 1 second.

It is an addition specific subject matter of the present invention anapparatus for implementing the method for determining the admittance ofan auditory canal according to any one of claims 1 to 7, that includes:

-   an impedence probe configured to be coupled with an auditory    canal (6) having    -   a box-like body with a first end configured to be coupled to the        auditory canal,    -   a speaker located close to a second end of the box-like body,        that is opposed to the first end, configured to emit an exciting        sound signal, the box-like body being sealed and containing        inside an air volume V_(probe) at atmospheric pressure,    -   a microphone array housed inside the box-like body and        configured to detect signals back from the auditory canal,        comprising at least a first microphone and at least a second        microphone placed between them at a distance Δx₁₂ that depends        on the frequencies of the exciting sound signal s(t), each        microphone being configured to directly detect a return sound        pressure p(x, t) as a function of time t and to output an        electrical signal r (x, t);-   a control and processing device configured to control and process    input and output signals of the impedance probe and to implement    step B to L, having:    -   a generation unit configured to generate a digital signal s(n)        and send it to the speaker through a D/A conversion board that        is removably coupled to the speaker, and    -   acquisition sound board configured to acquiring output signal        from microphone array, through an A/D conversion board that is        removably coupled to the microphone array (8),

    the impedance probe and the control and processing device being    removably coupled among them.

According to another aspect of the invention, said control andprocessing device can be further configured to implement said step M.

According to a further aspect of the invention, said box-like body canbe hollow cylindrical shaped.

According to an additional aspect of the invention, said apparatus canbe configured to input broadband exciting sound signal s(t) in afrequency range between 100 Hz to 5000 Hz and wherein the distance Δx₁₂is equal to 12 mm.

According to another aspect of the invention, said second end isprovided with an adapter configured to get easy coupling with theauditory canal, optionally said adapter being removable.

According to a further aspect of the invention, said adapter istruncated cone shaped, optionally made of rubber latex.

The advantages offered by the method according to the invention arenumerous and significant with respect to the solutions of the prior art.

The method of the present invention is not invasive because it allows tomeasure the acoustic admittance of the ear entry at ambient pressurewithout altering the static pressure of the patient’s auditory canal.Another advantage is that of allowing the analysis of the functionalstate of the ear over a wide range of frequencies. In addition, the useof the p-p (pressure-pressure) technique to measure the acousticadmittance of the ear allows the use of pressure probes, i.e.microphones, based on MEMS technology, thus lowering the productioncosts of impedance probes.

For measuring the acoustic admittance, which is the reciprocal of theacoustic impedance, in the auditory canal it is necessary to measure thepressure and velocity signals obtained as a response to a sound stimulussent at its entry. The p-p technique is based on the Euler acoustic waveequation which allows to reconstruct the velocity signal of an acousticparticle by numerically integrating the gradient signal of the acousticpressure over time. This choice, in many ways not optimal from ametrological point of view, was however satisfactory in the frequencyrange required by the application, and currently offers a significantreduction in the production costs of the probe, thanks to the commercialsuccess of the MEMS technologies applied to pressure microphones.

The present invention will be now described, by way of illustration andnot by way of limitation, according to its preferred embodiments, byparticularly referring to the Figures of the annexed drawings, in which:

FIG. 1 is a schematic block diagram of preferred embodiment of theapparatus for measuring the acoustic admittance of an ear according tothe invention;

FIG. 2 shows a flow chart of the algorithm executed by a first preferredembodiment of the method for measuring the acoustic admittance of an earaccording to the invention;

In the Figures, identical reference numerals will be used for alikeelements.

Due to the conformation of the auditory canal, the air is constrained tooscillate substantially along the longitudinal axis of the auditorycanal, therefore it is possible to approximate the velocity vector withthe velocity along the direction of said axis, which we set as the xdirection, or x axis.

With reference to FIG. 1 , an apparatus 100 comprises an impedance probe1 having a box-like body 2 with a first end 3 configured to be coupledwith an external measurement environment, which in the case shown inFIG. 1 is an external auditory canal 6 of a patient. The box-like body 2contains within it at least one speaker 4 placed in proximity to asecond end 5 thereof, opposite the first end 3.

The speaker 4 is configured to emit an acoustic signal s (t) for inputto the external auditory canal 6.

The box-like body 2 of the preferred embodiment of FIG. 1 has a hollowcylindrical shape. Conventionally, we set the position along the x axisof the first end 3, x_(a), as the origin of the x axis, x_(a) = 0.

The box-like body 2 is rigid, optionally made of brass or rigid resin.The box-like body 2 is sealed and contains within it a known air volumeV_(probe) at atmospheric pressure. The coupling of the impedance probe1, through its first end 3, with the external auditory canal of apatient is such that the air volume V_(probe) and an air volumeV_(canal) inside the auditory canal form an overall air volumeV_(overall) at atmospheric pressure, i.e. V_(overall) = V_(probe) +V_(canal). In other words, an overall volume V_(overall) containing airat atmospheric pressure is bounded by the impedance probe and theauditory canal when they are coupled together.

Furthermore, the coupling is such that the axis passing between thegeometric centres of the first end 3 and of the second end 5,conventionally the longitudinal axis of the impedance probe, isparallel, substantially coincident, with the axis of the auditory canal,that is the x axis.

In the preferred embodiment, the first end 3 is provided with an adapter7, optionally having a truncated cone shape and made of rubber latex,configured to facilitate coupling to the external auditory canal 6. Theadapter 7 can be removable to be replaced or cleaned before use ondifferent patients.

A microphone array 8, that is configured to detect return signals fromthe external auditory canal 6, is housed in the box-like body 2. Themicrophone array 8 comprises a first microphone 9 and at least onesecond microphone 10, each one configured to directly detect a returnacoustic pressure p(x, t) as a function of time t and provide with anelectrical signal r(x, t), at a first measurement point x₁ and at asecond measurement point x₂ respectively.

The distance Δx₁₂ between the first measurement point x₁ of the firstmicrophone 9 and the second measurement point x₂ of the secondmicrophone 10 depends on the frequency of the signal of interest.Ideally, for reconstructing the velocity signal from the pressuresignals measured by two microphones, a different distance Δx should beused for each frequency, that is for each wavelength, of the sound fieldto be measured.

In the preferred embodiment, an optimum distance is used to cover arange of working frequencies. To reduce the experimental error in thereconstruction of the velocity signal due to an error of the finitedifference approximation at high frequencies, the measured wavelengthmust be greater than about six times the distance between the twomicrophones. For example, for a distance between probes equal to 50 mm,the high frequency limit of the sound field beyond which theexperimental error significantly increases is 1.25 kHz, for a distanceequal to 12 mm the high frequency limit is 5 kHz, and for a distance of6 mm the high frequency limit is 10 kHz.

In the case of impedance measurement within an auditory canal of a humanear, the distance Δx₁₂ is equal to 12 millimeters to optimize themeasurement of acoustic admittance in the 100-5000 Hz frequency range.

In other embodiments of the present invention, the microphone array 8comprises a plurality of microphones greater than two, placed atdifferent distances Δx_(ij) from each other, so as to obtain a moreaccurate reconstruction of the velocity signal for each range offrequencies.

The minimum distance on the longitudinal axis between the second end 5and the central point x₀ of the microphone array 8, i.e. between thefirst and second measurement points x₁, x₂, must be such as to minimizethe microphone array 8 measurement error due to the proximity to a soundsource. In the preferred embodiment, this distance is equal to 35millimeters.

The impedance probe 1 is removably coupled with a control and processingdevice 11 configured to control and process the input and output signalsfrom the impedance probe 1. The control and processing device 11includes a generation unit 12 configured to generate a digital signals(n) and send it to the speaker 4, through a D/A conversion board 13which is removably connected to the speaker 4, and an acquisition soundboard 14 configured to acquire the output signals from the microphonearray 8, by means of an A/D conversion board 15, which is removablyconnected to the microphone array 8. The generation unit 12 and theacquisition sound board 14 are connected to each other. Optionally, thecontrol and processing device 11 can be removably connected to one ormore devices, such as for example PCs, smartphones and tablets, and/orto one or more screens 16 configured to display the signals controlledand processed by the control and processing device 11.

For the measurement of the acoustic admittance inside the auditory canalit is necessary to measure the acoustic pressure and particle velocitysignals in the air obtained as a response to a sound stimulus sent atits entry, that is to the input signal. The impedance probe 1, asdescribed in FIG. 1 , allows to indirectly measure the velocity signal,starting from two acoustic pressure signals detected at a distance Δxone from the other along the axis of the probe itself.

In fact, by considering a one-dimensional sound field in a medium withdensity ρ, then the Euler acoustic equation that links acoustic pressurep(x,t) and particle velocity v(x,t) to a point in the sound field can bewritten as:

$\begin{matrix}{\frac{\partial v\left( {x,t} \right)}{\partial t} = - \frac{1}{\rho}\frac{\partial p\left( {x,t} \right)}{\partial x}} & \text{­­­(Eq. 1)}\end{matrix}$

from which one can get the velocity signal by integrating:

$\begin{matrix}{v\left( {x,t} \right) = - \frac{1}{\rho}{\int_{- \infty}^{t}{\frac{\partial p\left( {x,t} \right)}{\partial x}dt}}} & \text{­­­(Eq. 2)}\end{matrix}$

By using the finite difference approximation method, the pressuregradient

$\frac{\partial p}{\partial x}$

can be estimated in practice by measuring the pressures at two closelyspaced points A and B separated by a distance Δx:

$\begin{matrix}{\frac{\partial p\left( {x,t} \right)}{\partial x} \approx \frac{p(B) - p(A)}{\Delta x}} & \text{­­­(Eq.3)}\end{matrix}$

Note that this approximation is valis only if Δx is small compared withthe shortest wave lengths in the measured sound field.

Substituting equation 3 into eq. 2, the velocity is calculated as:

$\begin{matrix}{v\left( {x,t} \right) = - \frac{1}{\rho\Delta x}{\int_{- \infty}^{t}{p(B) - p(A)\mspace{6mu}\, dt}}} & \text{­­­(Eq. 4)}\end{matrix}$

Similarly, the acoustic pressure can be estimated as the average of thepressures p(A) e p(B).

Therefore, for the impedance probe 1 it is possible it is possibleapproximating the spatial derivative of the pressure with itsincremental ratio Δx₁₂ and performing integration over time to obtainthe component of the velocity signal oriented along x direction:

$\begin{matrix}{v\left( {x,t} \right) \approx - \frac{1}{\rho\Delta x}\left\lbrack {\int_{- \infty}^{t}{p_{1}\left( {x_{1},\tau} \right) - p_{2}\left( {x_{2},\tau} \right)d\tau}} \right\rbrack} & \text{­­­(Eq. 5)}\end{matrix}$

And calculating the pressure as

$\begin{matrix}{p\left( {x,t} \right) = \frac{p_{1}\left( {x_{1},t} \right) + p_{2}\left( {x_{2},t} \right)}{2}} & \text{­­­(Eq. 6)}\end{matrix}$

with

$\begin{matrix}{p_{1}\left( {x_{1},t} \right) = p\left( {x_{0} + \frac{\Delta x}{2},t} \right)\text{e}p_{2}\left( {x_{2},t} \right) = p\left( {x_{0} - \frac{\Delta x}{2},t} \right)} & \text{­­­(Eq.7)}\end{matrix}$

where p₁(x₁, t) e p₂(x₂, t) are pressure signals measured through thefirst and the second microphone 9,10.

It is usual to refer the pressure and velocity signals, calculatedthrough the equations 5 to 7, to the central point x₀ between the twomicrophones, which is therefore the actual measuring point {p(x₀, t);v(x₀, t)}.

To identify a linear time-invariant acoustic system, such as theauditory canal of a human or mammalian ear, it is necessary to know itstransfer function by which it is possible to analyse its response tosound waves of any frequency. However, the study of impedance requiresknowledge of the two acoustic pressure and velocity responses in thefrequency domain. Thus, the most suitable stimulus to measure theseresponses must be able to excite the system in the whole range offrequencies that one plans to analyse, therefore this stimulus has to begenerated by a signal whose average temporal energy is the same for eachfrequency component. For this reason, the calculation of the admittanceis based on the impulse responses of the linear time-invariant acousticsystem calculated from the measurements of the field excited with chirpor sweep signal, that is a frequency modulated signal wherein theinstantaneous frequency varies linearly with time:

$\begin{matrix}{\delta(t) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r(t)} \right\}} \right\}} & \text{­­­(Eq. 8)}\end{matrix}$

where δ(t) is the impulse response, IFFT is the Inverse Fast FourierTransform, FFT(s′(t)) is the Fast Fourier Transform of the time-reversedsweep signal and FFT (r(t)) is the Fast Fourier Transform of the signalmeasured as the response of the system to the sweep signal stimulus.

The flow chart of a preferred embodiment of the method for measuring theacoustic admittance of an ear is shown in FIG. 2 inside the blockcorresponding to the control and processing device 11 of the apparatus100 of FIG. 1 ;

Preliminarily, the impedance probe 1 is calibrated, for example with astandard calibration method used in tympanometry to calculate the airvolume V_(probe) contained within impedance probe 1. For example, aclassic method is the one known to those skilled in the art which usesthree known volumes of air of 0.2 cc, 2 cc and 4 cc.

In an initial step, the calibrated impedance probe 1 is coupled to anauditory canal 6 through a first end (3) thereof in a couplingconfiguration (Q) such that its air volume V_(probe) and an air volumeV_(canal) inside the auditory canal form an overall air volumeV_(overall), that is V_(complessivo)= V_(sonda)+ V_(canale), and thatits longitudinal axis is parallel, substantially coincident, with theaxis of the auditory canal, that is the x axis.

Coupling configuration means the mutual and contact placement of theimpedance probe 1 and auditory canal 6.

In step 200, the generation unit 12 generates, in other termssynthesizes, an excitation chirp or sweep signal s(n) which varies infrequency from a minimum frequency F_(min) greater than 100 Hz to amaximum frequency F_(max) less than 5000 Hz, in a time T_(sweep) whichis less than 10 seconds, optionally equal to 2 seconds, even moreoptionally equal to 1 second. In the preferred embodiment, theexcitation sweep signal s (n) is a logarithmic or linear sinusoidalsweep signal. The signal s (n) is sent from the control and processingdevice 11 to the speaker 4 after being converted in a step 210 into ananalog signal s(t) via the D/A conversion board 13.

In step 220, a first acoustic pressure p₁(t) and a second acousticpressure p2(t) are directly measured at points x₁ and x₂ by the firstand second microphones 9, 10 which output a first electrical signalr₁(t) and a second electrical signal r₂(t) as a function of time,respectively. In a step 230, the control and processing device 11through the acquisition sound board 14 acquires the first and secondelectrical signals r₁(t), r₂(t) and converts them through the A/Dconversion board 15 into corresponding discretized values r₁(n) andr₂(n), where ∈[1; N], N∈N.

The A/D conversion board 15 is synchronized with the D/A conversionboard 13, so that the electrical signals r₁(t), r₂(t) are aligned intime, i.e. they are acquired in synchrony with the excitation signal s(t). In other words, the acquisition of electrical signals must beginexactly at the instant in which the excitation signal is emitted.

The number N of discretized values depends on the measurement sampling,i.e. on the temporal resolution of the acquisitions and therefore on theexcitation chirp or sweep signal s(n) synthesized in the step 200.

In step 240, the control and processing device 11 calculates the impulseresponse of the auditory canal 6 according to equation 8, obtaining afirst impulse response

δ₁^(au)(n)

and a second impulse response

δ₂^(au)(n)

from the first and second microphones 9 and 10, respectively:

$\begin{matrix}\begin{array}{l}{\delta_{1}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(n)} \right\}}{FFT\left\{ {r_{1}(n)} \right\}} \right\}} \\{\delta_{2}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(n)} \right\}}{FFT\left\{ {r_{2}(n)} \right\}} \right\}}\end{array} & \text{­­­(Eq. 9)}\end{matrix}$

The control and processing device 11 processes such impulsive responses

δ₁^(au)(n)

and

δ₂^(au)(n),

in step 250, to calculate a acoustic pressure impulse response

δ_(p)^(au)(n)

and a velocity impulse response

δ_(v)^(au)(n)

of the air particle at the measuring point x₀ according to equations 5and 6 respectively:

$\begin{matrix}\begin{matrix}{\delta_{v}^{au}(n) = \frac{\delta_{1}^{au}(n) - \delta_{2}^{au}(n)}{\left( {\rho \cdot \Delta\text{x}_{12}} \right)} + \delta_{v}^{au}\left( {n - 1} \right)} \\{\delta_{p}^{au}(n) = \frac{\delta_{1}^{au}(n) + \delta_{2}^{au}(n)}{2}}\end{matrix} & \text{­­­(Eq. 10)}\end{matrix}$

The apex “au” of the pressure and velocity impulsive responses indicatesthat they are quantities in arbitrary units [a.u.].

The pressure and velocity impulse responses

δ_(p)^(au)(n),

δ_(v)^(au)(n)

are then converted by the control and processing device 11 into physicalunits, multiplying each by a calibration constant α and β respectively,in step 260, in order to represent them in the respective physicalscales of [Pa] and [m/s]:

$\begin{matrix}\begin{array}{l}{\delta_{p}(n)\left\lbrack {Pa} \right\rbrack = \alpha\frac{\left\lbrack {Pa} \right\rbrack}{\left\lbrack {a.u.} \right\rbrack} \cdot \delta_{p}^{au}(n)\left\lbrack {a.\mspace{6mu} u.} \right\rbrack} \\{\delta_{v}(n)\left\lbrack {Pa} \right\rbrack = \beta\frac{\left\lbrack {m/s} \right\rbrack}{\left\lbrack {a.u.} \right\rbrack} \cdot \delta_{v}^{au}(n)\left\lbrack {a.\mspace{6mu} u.} \right\rbrack}\end{array} & \text{­­­(Eq. 11)}\end{matrix}$

The calibration constants α and β are obtained by means of methods knownin literature, see for example Stanzial D., Graffigna C. E., Protocollodi calibrazione in ampiezza e fase per sonde pressione-velocità in uncampo di riferimento a onde piane progressive, Associazione Italiana diAcustica, 44° Convegno Nazionale, Pavia, 7-9 giugno 2017 ISBN:978-88-88942-54-4 wherein the velocity signal is the one reconstructedfrom the pressure signals of the microphones. In other words, both thesignal p(x₀), and the signal v(x₀), which are reconstructed from thesignals p(x₁) and p₂(x₂) are calibrated. In further embodiments of thisinvention, the calibration constants α and β are provided by themanufacturers of microphonic probes.

Once the pressure and velocity impulsive responses have been convertedinto physical units, the Fast Fourier Transform is applied to them instep 270, by the control and processing device 11, to obtain thefrequency spectra P̂*(ω_(m)) and V̂*(ω_(m)) of the pressure and velocityimpulsive responses:

$\begin{matrix}\left\{ \begin{array}{l}{\hat{V}\text{*}\left( \omega_{m} \right) = FFT\left\{ {\delta_{v}(n)} \right\}} \\{\hat{P}\text{*}\left( \omega_{m} \right) = FFT\left\{ {\delta_{p}(n)} \right\}}\end{array} \right) & \text{­­­(Eq. 12)}\end{matrix}$

where ω_(m) is the discretized frequency with m∈[1; N/2]and the accent

$\hat{\mspace{6mu}}$

indicates that the spectra are complex. The apex * indicates that theyare spectra of the impulse responses of measured signals, i.e. thespectra are not calibrated. This is because microphones generally havedifferent responses at different frequencies.

Then, in step 280, a not yet calibrated admittance Ŷ*(ω_(m)) iscalculated by the control and processing device 11 as the ratio betweenthe cross spectrum Ĝ_(pv)(ω_(m)) of the impulse response spectrum ofacoustic pressure and of the impulse response spectrum of acousticvelocity, and the auto spectrum Ĝ_(pp)(ω_(m)) of the impulse responsespectrum of the of the acoustic pressure:

$\begin{matrix}{\hat{Y}\text{*}\left( \omega_{m} \right) = \frac{{\hat{G}}_{pv}\left( \omega_{m} \right)}{{\hat{G}}_{pp}\left( \omega_{m} \right)} = \frac{{\hat{V}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}{{\hat{P}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}} & \text{­­­(Eq. 13)}\end{matrix}$

Finally, in step 290, a measurement of the calibrated admittance of theauditory canal 6 is obtained by means of the control and processingdevice 11 as:

$\begin{matrix}{\hat{Y}\left( \omega_{m} \right) = \Gamma\left( \omega_{m} \right) \cdot {\hat{Y}}^{\ast}\left( \omega_{m} \right)} & \text{­­­(Eq. 14)}\end{matrix}$

where Γ(ω_(m)) is a frequency calibration curve obtained according to anamplitude and phase calibration protocol developed by the inventors forpressure-velocity probes in a plane wave reference field (Stanzial D.,Graffigna C. E., Funzione di calibrazione in ampiezza e fase per sondepressione-velocità ottenuta in campi di riferimento reattivi,Associazione Italiana di Acustica, 45° Convegno Nazionale, Aosta, 20-22giugno 2018 ISBN: 978-88-88942-56-8), wherein the velocity signal is theone reconstructed from the pressure signals of the microphonic probes.

The calibration curve Γ(ω_(m)) takes in account of different responsesof the microphonic probes as the frequency varies. In furtherembodiments of the present invention, calibration functions Γ(ω_(m)) areprovided by the manufacturers of the microphonic probes.

The admittance spectrum Ŷ(ω_(m)), magnitude and phase, is optionallydisplayed at the output on a screen 16 in step 295.

From the acoustic admittance Ŷ(ω_(m)) all the classical tympanometricparameters can be evaluated, such as for example the tympanic rigidity,the volume of the auditory canal. In other words, the parameters usedfor the audiometric clinical investigation are calculated from theresonance curve obtained through the admittance measured in the auditorycanal.

The separation of the volume of air inside the auditory canal from thatinside the probe, the absorption of energy by the eardrum and otherparameters useful for diagnostic purposes are obtained from the analysisof the shape of the resonance curve of the acoustic admittance.

In the embodiments of the method according to the invention in which themicrophonic probe 8 includes a plurality of microphonic probes greaterthan two, the steps described above are performed for each combinationof pair of microphones having distances Δx_(ij) that are different fromeach other. Subsequently, the frequency admittance curve would berecomposed for each processed frequency segment.

Since it is very important that the coupling of the impedance probe 2with the auditory canal 6 is proper, i.e. that the volume V_(probe) ofthe air of the impedance probe 2 and the volume V_(canal) of the airinside the auditory canal 6 form an overall volume V_(overall) of air atatmospheric pressure, optionally a preliminary procedure is performed toidentify the right coupling of the probe to the auditory canal. Thispreliminary procedure is based on the identification of the resonance ofthe admittance. Once the resonance has been identified, a more accurateadmittance measurement is carried out to obtain the clinical-audiometricparameters while maintaining the coupling of the impedance probe withthe auditory canal for which the resonance occurs.

This preliminary procedure includes carrying out the method wherein animpedance probe (1) is coupled, for example from an operator, with theauditory canal (6) of a patient in a first coupling configuration (Q1)and wherein a fast sweep signal s^(fast)(t), with

T_(sweep)^(fast)

less than one second, optionally equal to about half a second. Theabove-described steps of the method are performed to obtain a firstcalibrated admittance Ŷ₁(ω_(m)) of the auditory canal 6.

It is checked whether a resonance condition in the admittance issatisfied, that is if the peak of the admittance module corresponds withthe zero-crossing of its phase.

If a resonance condition occurs, the first coupling configuration (Q1)corresponds to the right coupling of the probe with the auditory canaland the said procedure ends.

The method is then executed keeping the first coupling configuration(Q1) and sending a slow sweep signal s(t), i.e., with T_(sweep) lessthan 10 seconds and greater or equal to 1 second for measuring thecalibrated Ŷ(ω_(m)) from which obtaining the audiometric parameters ofthe auditory canal.

If a resonance condition does not occur, then the method is iterated ina second coupling configuration (Q2) that is different in respect to theprevious one (Q1), for example the operator changes the mutual andplacement of probe and auditory canal, and with the fast sweep signals^(fast)(t) obtaining a second calibrated admittance Ŷ₂(ω_(m)) and thechecking is iterated.

In other words, a resonance in the admittance spectrum implies that theimpedance probe is positioned correctly in respect to the ear and a moreaccurate measurement can be performed for determining the admittance.

In a preferred embodiment of the method, if the resonance condition isverified, the control and processing device 11 alerts an operator, forexample by emitting a sound signal or by sending a signal on a screen,so that the configuration of coupling for which resonance occurs can bekept while performing the final measurement. The preferred embodimentsof this invention have been described, but it must be understood thatthose skilled in the art can make other variations and changes, withoutso departing from the scope of protection thereof, as defined by theattached claims.

1. A method for determining the admittance of an auditory canal forclinical-audiometric investigations, the method comprising at least oneor more iterations of a procedure, in which each iteration is associatedwith a respective coupling configuration between an impedance probe andthe auditory canal, in which said procedure includes the followingsteps: A. coupling the calibrated sealed impedance probe having a knownair volume V_(probe), through a first end thereof, with the auditorycanal so that: the air volume Vpro_(b)e sealed inside the impedanceprobe and the air volume V_(canal) inside the auditory canal form anoverall air volume V_(overall), and a longitudinal axis of the impedanceprobe is substantially coincident to a longitudinal axis of the auditorycanal; B. sending a broadband exciting sound signal s(t) to the auditorycanal through a speaker of the impedance probe, such speaker beinglocated at a second end of the impedance probe opposed to the first end;C. directly detecting an acoustic pressure p₁(t), p₂(t) back from theauditory canal in at least two points x₁ and x₂, respectively, locatedalong the longitudinal axis of the impedance probe at distance Δx₁₂between them, by means of a microphone array that is included in theimpendence probe and outputs electric signals r₁(t) and r₂(t); D.acquiring and discretizing the output electric signals r₁(t) and r₂(t)from the microphone array, obtaining discretized signals r₁(n) and r₂(n)respectively, with n∈ [1; N], N∈ ℕ; E. calculating a first impulseresponse δ₁^(au)(n) and a second impulse response δ₂^(au)(n) by thefollowing equations:$\delta_{1}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{1}(t)} \right\}} \right\}$$\delta_{2}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{2}(t)} \right\}} \right\}$where s′(t) is the time reversed broadband sound signal s(t), FFT is aFast Fourier Transform and IFFT is an inverse FFT; F. calculating animpulse response δ_(p)^(au)(n) of the acoustic pressures p ₁(t) andp₂(t) and an impulse response δ_(v)^(au)(n) of velocity of an airparticle at measurement point x ₀ along the longitudinal axis of theimpedance probe:$\delta_{v}^{au}(n) = \frac{\delta_{1}^{au}(n) - \delta_{2}^{au}(n)}{\left( {\rho \cdot \Delta\text{x}_{12}} \right)} + \delta_{v}^{au}\left( {n - 1} \right)$$\delta_{p}^{au}(n) = \frac{\delta_{1}^{au}(n) + \delta_{2}^{au}(n)}{2}$such a measurement point x ₀ being a centre point between points x₁ andx₂; G. converting the impulse responses δ_(p)^(au)(n),δ_(v)^(au)(n) ofpressure and velocity to pressure and velocity physical units bymultiplying each one by a calibration constant α and β known a priori,respectively, as follows: δ_(p)(n)[Pascal] = α ⋅ δ_(p)^(au)(n)δ_(v)(n)[Pascal meter/second] = β ⋅ δ_(v)^(au)(n) H. calculatingfrequency spectra P̂*(ω_(m)), V̂*(ω_(m)) of the impulse responses ofpressure and velocity respectively, through Fast Fourier Transform, asfollows: $\left\{ \begin{array}{l}{{\hat{V}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{v}(n)} \right\}} \\{{\hat{P}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{p}(n)} \right\}}\end{array} \right)$ where ω _(m) is a discretized frequency with m∈ [1;N/2]; I. calculating an admittance Ŷ*(ω_(m)) as a ratio between a crossspectrum Ĝ_(pv) (ω_(m)) of the spectrum of the acoustic pressure impulseresponse and of the spectrum of the acoustic velocity impulse response,and an auto spectrum Ĝ_(pp)(ω_(m)) of the spectrum of the acousticpressure impulse response:${\hat{Y}}^{\ast}\left( \omega_{m} \right) = \frac{{\hat{G}}_{pv}\left( \omega_{m} \right)}{{\hat{G}}_{pp}\left( \omega_{m} \right)} = \frac{{\hat{V}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}{{\hat{P}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}$L. obtaining a calibrated frequency spectrum Ŷ(ω_(m)) of the admittancethrough a calibration function Γ(ω_(m)) known a priori, according to theequation: Ŷ(ω_(m)) = Γ(ω_(m)) ⋅ Ŷ^(*)(ω_(m))  , said steps D to L beingperformed by a control and processing device.
 2. The method according toclaim 1, wherein the calibration constant α and β, and the calibrationfunction (ω_(m)) are known a priori .
 3. The method according to claim1, wherein the exciting sound signal s(t) is a sweep signal, varyingfrom a minimum frequency F_(min) greater than 100 Hz to a maximumfrequency F_(max) less then 5000 Hz over a time T_(sweep) less than 10seconds .
 4. The method according to claim 3, wherein the distance Δx₁₂is equal to 12 mm.
 5. The method according to claim 3, wherein step Bcomprises the substeps: B.1 synthesizing a digital sweep signal s(n) bya signal generator, B.2 converting the digital sweep signal s(n) into abroadband exciting sound signal s(t) to be input to the speaker througha D/A converter.
 6. The method according to claim 5, wherein step D isimplemented through an A/D converter synchronized with the D/Aconverter.
 7. The method according to claim 1, wherein the calibratedfrequency spectrum Ŷ(ω_(m)) of the admittance is further input to adisplay to be displayed.
 8. The method according to claim 1, wherein theimpedance probe and the auditory canal are coupled into a first couplingconfiguration, the exciting sound signal s(t) is a fast sweep signals^(fast)(t) varying in frequency over a time T_(sweep)^(fast) less thanone second, to obtain a first calibrated admittance Ŷ ₁(ω_(m)) and saidprocedure comprises further the additional step: M. checking whether aresonance condition in the calibrated admittance Ŷ₁(ω_(m)) is satisfied,thereby a peak of the module of the first calibrated admittanceŶ₁(ω_(m)) corresponds to zero-crossing of its phase, and wherein: if aresonance condition does not occur, another iteration of said procedurecomprising steps A to M is implemented, wherein the impedance probe andthe auditory canal are coupled in another coupling configuration that isdifferent from the previous coupling configuration and the excitingsound signal s(t) is the fast sweep signal s^(fast)(t); if a resonancecondition occurs, steps B to L of such procedure are implemented,wherein the coupling configuration is the one for which the resonancecondition occurs and the exciting sound signal s(t) is a sweep signalvarying in frequency over a time greater than the time T_(sweep)^(fast)of the fast sweep signal s ^(fast)(t), and said one or more iterationsof said procedure end.
 9. A clinical-audiometric investigation methodcomprising a method for determining the admittance of an auditory canalcomprising at least one or more iterations of a procedure, in which eachiteration is associated with a respective coupling configuration betweena impedance probe and the auditory canal, in which said procedureincludes the following steps: A. coupling the calibrated sealedimpedance probe having a known air volume V_(probe), through a first endthereof, with the auditory canal so that: the air volume V_(probe)sealed inside the impedance probe and the air volume V_(canal) insidethe auditory canal form an overall air volume V_(overall), and alongitudinal axis of the impedance probe is substantially coincident toa longitudinal axis of the auditory canal; B. sending a broadbandexciting sound signal s(t) to the auditory canal through a speaker ofthe impedance probe, such speaker being located at a second end of theimpedance probe opposed to the first end; C. directly detecting anacoustic pressure p₁(t), p₂(t) back from the auditory canal in at leasttwo points x₁ and x₂ respectively located along the longitudinal axis ofthe impedance probe at distance Δx₁₂ between them, by means of amicrophone array that is included in the impendence probe and outputselectric signals r₁ (t) and r₂(t); D. acquiring and discretizing theoutput electric signals r₁(t) and r₂(t) from the microphone array,obtaining discretized signals r₁(n) and r₂(n) respectively, with n∈ [1;N], N∈ ℕ; E. calculating a first impulse response δ₁^(au)(n) and asecond impulse response δ₂^(au)(n) by the following equations:$\delta_{1}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{1}(t)} \right\}} \right\}$$\delta_{2}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{2}(t)} \right\}} \right\}$where s′(t) is the time reversed broadband sound signal s(t), FFT is aFast Fourier Transform and IFFT is an inverse FFT; F. calculating animpulse response δ_(p)^(au)(n) of the acoustic pressures p ₁(t) andp₂(t) and an impulse response δ_(v)^(au)(n) of velocity of an airparticle at measurement point x ₀ along the longitudinal axis of theimpedance probe:$\delta_{v}^{au}(n) = \frac{\delta_{1}^{au}(n) - \delta_{2}^{au}(n)}{\left( {\rho \cdot \Delta\text{x}_{12}} \right)} + \delta_{v}^{au}\left( {n - 1} \right)$such a measurement point x ₀ being a centre point between points x₁ andx₂; G. converting the impulse responses δ_(p)^(au)(n),δ_(v)^(au)(n) ofpressure and velocity to pressure and velocity physical units bymultiplying each one by a calibration constant α and β known a priori,respectively, as follows: δ_(p)(n)[Pascal] = α ⋅ δ_(p)^(au)(n)δ_(v)(n)[Pascal meter/second] = β ⋅ δ_(v)^(au)(n) H. calculatingfrequency spectra P̂*(ω_(m)),V̂*(ω_(m)) of the impulse responses ofpressure and velocity respectively, through Fast Fourier Transform, asfollows: $\left\{ \begin{array}{l}{{\hat{V}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{v}(n)} \right\}} \\{{\hat{P}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{p}(n)} \right\}}\end{array} \right)$ where ω _(m) is a discretized frequency with m∈ [1;N/2]; I. calculating an admittance Ŷ*(ω_(m)) as a ratio between a crossspectrum Ĝ_(pv)(ω_(m)) of the spectrum of the acoustic pressure impulseresponse and of the spectrum of the acoustic velocity impulse response,and an auto spectrum Ĝ_(pp)(ω_(m)) of the spectrum of the acousticpressure impulse response:${\hat{Y}}^{\ast}\left( \omega_{m} \right) = \frac{{\hat{G}}_{pv}\left( \omega_{m} \right)}{{\hat{G}}_{pp}\left( \omega_{m} \right)} = \frac{{\hat{V}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}{{\hat{P}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}$L. obtaining a calibrated frequency spectrum Ŷ(ω_(m)) of the admittancethrough a calibration function Γ(ω_(m)) known a priori, according to theequation: Ŷ(ω_(m)) = Γ(ω_(m)) ⋅ Ŷ^(*)(ω_(m))  , said steps D to L beingperformed by a control and processing device, wherein the investigationis implemented in the coupling configuration of the last one or moreiterations and wherein the exciting sound signal s(t) varies infrequency over a time grater or equal to 1 second.
 10. An apparatusconfigured to executed a method for determining the admittance of anauditory canal clinical-audiometric investigations comprising at leastone or more iterations of a procedure, in which each iteration isassociated with a respective coupling configuration between an impedanceprobe and the auditory canal, in which said procedure includes thefollowing steps: A. coupling the calibrated sealed impedance probehaving a known air volume V_(probe), through a first end thereof, withthe auditory canal so that: the air volume V_(probe) sealed inside theimpedance probe and the air volume V_(canal) inside the auditory canalform an overall air volume V_(overall), and a longitudinal axis of theimpedance probe is substantially coincident to a longitudinal axis ofthe auditory canal; B. sending a broadband exciting sound signal s(t) tothe auditory canal through a speaker of the impedance probe, suchspeaker being located at a second end of the impedance probe opposed tothe first end; C. directly detecting an acoustic pressure p₁(t), p₂(t)back from the auditory canal in at least two points x₁ and x₂,respectively, located along the longitudinal axis of the impedance probeat distance Δx₁₂ between them, by means of a microphone array that isincluded in the impendence probe and outputs electric signals r₁(t) andr₂(t); D. acquiring and discretizing the output electric signals r₁(t)and r₂(t) from the microphone array, obtaining discretized signals r₁(n)and r₂(n) respectively, with n∈ [1; N], N∈ ℕ; E. calculating a firstimpulse response δ₁^(au)(n) and a second impulse response δ₂^(au)(n) bythe following equations:$\delta_{1}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{1}(t)} \right\}} \right\}$$\delta_{2}^{au}(n) = IFFT\left\{ \frac{FFT\left\{ {s^{\prime}(t)} \right\}}{FFT\left\{ {r_{2}(t)} \right\}} \right\}$where s′(t) is the time reversed broadband sound signal s(t), FFT is aFast Fourier Transform and IFFT is an inverse FFT; F. calculating animpulse response δ_(p)^(au)(n) of the acoustic pressures p ₁(t) andp₂(t) and an impulse response δ_(v)^(au)(n) of velocity of an airparticle at measurement point x ₀ along the longitudinal axis of theimpedance probe:$\delta_{v}^{au}(n) = \frac{\delta_{1}^{au}(n) - \delta_{2}^{au}(n)}{\left( {\rho \cdot \Delta\text{x}_{12}} \right)} + \delta_{v}^{au}\left( {n - 1} \right)$$\delta_{p}^{au}(n) = \frac{\delta_{1}^{au}(n) + \delta_{2}^{au}(n)}{2}$such a measurement point x ₀ being a centre point between points x₁ andx₂; G. converting the impulse responses δ_(p)^(au)(n), δ_(v)^(au)(n) ofpressure and velocity to pressure and velocity physical units bymultiplying each one by a calibration constant α and β known a priori,respectively, as follows: δ_(p)(n)[Pascal] = α ⋅ δ_(p)^(au)(n)δ_(v)(n)[Pascal meter/second] = β ⋅ δ_(v)^(au)(n) H. calculatingfrequency spectra P̂*(ω_(m)),V̂*(ω_(m)) of the impulse responses ofpressure and velocity respectively, through Fast Fourier Transform, asfollows: $\left\{ \begin{array}{l}{{\hat{V}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{v}(n)} \right\}} \\{{\hat{P}}^{\ast}\left( \omega_{m} \right) = FFT\left\{ {\delta_{p}(n)} \right\}}\end{array} \right)$ where ω _(m) is a discretized frequency with m∈ [1;N/2]; I. calculating an admittance Ŷ*(ω_(m)) as a ratio between a crossspectrum Ĝ_(pv)(ωm) of the spectrum of the acoustic pressure impulseresponse and of the spectrum of the acoustic velocity impulse response,and an auto spectrum Ĝ_(pp)(ω_(m)) of the spectrum of the acousticpressure impulse response:${\hat{Y}}^{\ast}\left( \omega_{m} \right) = \frac{{\hat{G}}_{pv}\left( \omega_{m} \right)}{{\hat{G}}_{pp}\left( \omega_{m} \right)} = \frac{{\hat{V}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}{{\hat{P}}^{\ast}\left( \omega_{m} \right) \cdot {\hat{P}}^{\ast}\left( \omega_{m} \right)}$L. obtaining a calibrated frequency spectrum Ŷ(ω_(m)) of the admittancethrough a calibration function Γ(ω_(m)) known a priori, according to theequation: Ŷ(ω_(m)) = Γ(ω_(m)) ⋅ Ŷ^(*)(ω_(m))  , said apparatusincluding; an impedance probe configured to be coupled with an auditorycanal having a box-like body with a first end configured to be coupledto the auditory canal, a speaker located close to a second end of thebox-like body, that is opposed to the first end, configured to emit anexciting sound signal, the box-like body being sealed and containinginside an air volume V_(probe) at atmospheric pressure, a microphonearray housed inside the box-like body and configured to detect signalsback from the auditory canal, comprising at least a first microphone andat least a second microphone placed between them at a distance Δx₁₂ thatdepends on the frequencies of the exciting sound signal s(t), eachmicrophone being configured to directly detect a return sound pressurep(x, t) as a function of time t and to output an electrical signal r (x,t); a control and processing device configured to control and processinput and output signals of the impedance probe and to implement step Bto L, having: a generation unit configured to generate a digital signals(n) and send it to the speaker through a D/A conversion board that isremovably coupled to the speaker, and an acquisition sound boardconfigured to acquiring output signal from microphone array, through anA/D conversion board that is removably coupled to the microphone array,the impedance probe and the control and processing device beingremovably coupled among them.
 11. The apparatus according to claim 10,wherein the apparatus is configured to execute a first iteration of saidprocedure associated with a first coupling configuration between theimpedance probe and the auditory canal and said exciting sound signals(t) is a fast sweep signal s^(fast)(t) varying in frequency over a timeT_(sweep)^(fast) less than one second to obtain a first calibratedadmittance Ŷ ₁(ω_(m)) and the control and processing device is furtherconfigured to execute the following step M for each one of said one ormore iteration: M. checking whether a resonance condition in thecalibrated admittance Ŷ₁(ω_(m)) is satisfied, thereby a peak of themodule of the first calibrated admittance Ŷ₁(ω_(m)) corresponds tozero-crossing of its phase, and wherein: if a resonance condition doesnot occur, another iteration of said procedure comprising steps A to Mis implemented, wherein the impedance probe and the auditory canal arecoupled in another coupling configuration that is different from theprevious coupling configuration and the exciting sound signal s(t) isthe fast sweep signal s^(fast)(t); if a resonance condition occurs,steps B to L of such procedure are implemented, wherein the couplingconfiguration is the one for which the resonance condition occurs andthe exciting sound signal s(t) is a sweep signal varying in frequencyover a time greater than the time T_(sweep)^(fast) of the fast sweepsignal s ^(fast)(t), and said one or more iterations of said procedureend.
 12. The apparatus according claim 10, wherein the box-like body ishollow cylindrical shaped.
 13. The apparatus according claim 10,configured to input broadband exciting sound signal s(t) in a frequencyrange between 100 Hz to 5000 Hz and wherein the distance Δx₁₂ is equalto 12 mm.
 14. The apapratus according to claim 10, wherein the secondend is provided with an adapter configured to get easy coupling with theauditory canal.
 15. The apparatus according to claim 14, wherein theadapter is truncated cone shaped.
 16. The method according to claim 1,wherein the calibration constant α and β, and the calibration functionΓ(ω_(m)) are provided by manufacturer of microphones.
 17. The methodaccording to claim 3, wherein the sweep signal is a linear orlogarithmic sinusoidal signal.
 18. The method according to claim 3,wherein the time T_(sweep) is equal to 2 seconds.
 19. The methodaccording to claim 3, wherein the time T_(sweep) is equal to 1 seconds.20. The apparatus according to claim 14, wherein said adapter isremovable.
 21. The apparatus according to claim 14, wherein said adapteris made of rubber latex.